Modern commercial production of ethylene oxide is accomplished by the vapor phase reaction of ethylene with molecular oxygen in the presence of a silver-containing catalyst. Suitable silver-containing catalysts comprise silver, at least partially in the elemental form, in a finely divided state, distributed throughout as well as on the surface of a porous refractory support material. Because of the importance of ethylene oxide as a tonnage chemical of commerce, substantial effort has been directed toward improving the performance of the catalysts used in this process, since the performance of the catalyst has a major impact on the over-all economics of the ethylene oxide production process.
The performance of an ethylene oxide catalyst, although conventionally equated with selectivity (selectivity being defined as the molar ratio of ethylene oxide formed per mole of ethylene reacting, usually expressed as a percent of the theoretical--the theoretical maximum being 100%), involves a plurality of additional factors. A catalyst should be capable of providing high ethylene conversions per pass in order to simplify the recovery of ethylene oxide from the reactor effluent (ethylene conversion per pass being defined as the ratio of the moles of ethylene reacted within the reactor or reactors to the number of moles of ethylene contained in the reactor feed gas, computed without regard to whether the ethylene is obtained as fresh feed or from recycle streams, again usually expressed as a percent of the theoretical maximum of 100%). The catalyst, additionally, should be active, particularly at relatively low reaction temperatures, and should be long-lived since replacement of the catalyst usually mandates interruption of commercial production for catalyst replacement. Activity at low temperature is a desirable feature because the lower the reaction temperature required with a fresh charge of catalyst, the greater is the room for temperature increase to compensate for deterioration of the catalyst with time. This is particularly significant when it is realized that commercial production of ethylene oxide at reactor inlet temperatures above about 300.degree. C. is more difficult than is such production at lower reactor inlet temperatures due, among other considerations, to a tendency for thermal decomposition of the desired ethylene oxide product. Operation at temperatures in the 200.degree. - 300.degree. C. region provides substantial margin for variation in reactor operating conditions, while operation above about 300.degree. C. provides less flexibility to safely compensate for such variations.
In view of these factors, much effort has been devoted to the improvement of ethylene oxide catalysts over the years and substantial attention has been devoted to catalyst preparation procedures as well as to the development of additives which would enhance the performance of ethylene oxide catalysts. Among the additives previously disclosed as having utility in the obtaining of enhanced performance are the alkaline earth metals (see U.S. Pat. No. 3,725,307) and the alkali metals (see U.S. Pat. No. 3,563,914). The latter patent emphasizes the importance of adding the alkali metal promoter to the catalyst support prior to the deposition of the silver and clearly shows that catalysts of inferior performance are obtained when it is attempted to add an alkali metal promoter to a catalyst already containing silver.
More recent work has emphasized the efficacy of alkali metals, particularly potassium, rubidium, and cesium, as promoters for ethylene oxide catalysts. For example, U.K. Patent Specification No. 1,413,251, published Nov. 12, 1975, discloses the obtaining of catalysts of enhanced selectivity by the incorporation of from 3.5 .times. 10.sup.-4 to 3 .times. 10.sup.-3 gram equivalent weights of potassium, rubidium, and/or cesium per kilogram of total catalyst, provided that these promoters are coincidentally deposited with the silver upon the support, and U.S. Pat. No. 3,962,136 makes exactly the same point with respect to coincidental deposition but teaches a broader range of alkali metal promoter to be employable. However, the temperatures required to obtain commercially satisfactory conversions with such catalysts are also high. In short, these catalysts, while selective, seem to have relatively low activity.
Another illustration of the use of alkali metals as promoters is provided in Belgian Pat. No. 821,439, granted on Apr. 24, 1975. The process of this patent, however, like that of U.S. Pat. No. 3,563,914, requires that the alkali metal be deposited upon the support prior to the deposition upon the support of the silver. Again, catalysts of good selectivity but poor activity characteristics are obtained. Additionally, it is difficult to see how the process of this Belgian patent significantly differs from the process of the U.K. patent specification discussed above since the high solubility of alkali metal compounds and the relatively low solubility of many silver compounds suggests that the alkali metal compound would be leached from the support and then redeposited thereon when, subsequently, the silver is introduced upon the support. Thus, it does not seem practicable to distinguish between the process of the Belgian patent and the process of the British patent specification, and a comparison of the selectivity-activity performance of catalysts prepared by both procedures would appear to confirm this.
Processes for producing alkali metal-containing catalysts, as set forth in the patents discussed above, present difficulties when applied to the manufacture of large batches of catalysts needed for commercial ethylene oxide production facilities. The amount of alkali metal is required to be controlled within relatively narrow limits. Such control is difficult to provide in commercial catalyst production, which usually is a batch operation. Accordingly, repeated analyses are required to ensure that the desired, critical, alkali metal content is attained. Also, techniques are required for the leaching of excess quantities of alkali metal which may be deposited, either through inadvertence or because of differences in absorbtivity between silver ion and alkali metal ion. Correspondingly, the catalyst preparation process becomes more complex and expensive than would otherwise be desirable.
Attempts to date made to overcome the drawbacks referred to above are exemplified by Belgian Pat. No. 822,857, granted June 2, 1975, but have also presented substantial difficulty. The promoter systems of this patent are extremely complex, involving combinations of up to 20 different materials. The complexities associated with the use of such promoter systems, in defined quantities, are obvious. Additionally, the catalyst preparation process of this reference requires use of alkaline solutions for impregnation of the support, which in turn mandates either the preparation of a coated, as opposed to an impregnated, catalyst or requires the use of silver-complexing agents to solubilize the silver compound if a true impregnated catalyst is desired. Coated catalysts are generally recognized today as being less satisfactory than impregnated catalysts because, among other factors, the silver in a coated catalyst would appear, in commercial use, to be somewhat more prone to separate, by abrasion or otherwise, from the support.
The terms "impregnated catalyst" and "coated catalyst" have artrecognized meanings. The former are prepared by impregnation techniques while the latter are generally prepared by mechanical mixing and/or spraying techniques; see Emmet, ed., "Catalysis" Vol. I, Reinhold Pub. Co., New York (1954), at pages 247-248, for general descriptions of these techniques.